The present invention relates to a forced oscillatory flow type heat pipe for transferring heat generated from a heating element such as a semiconductor element by an oscillatory flow of a charged liquid. Particularly, it relates to a forced oscillatory flow type heat pipe in which a ratio (motion coefficient) of the heat transport capacity to the oscillating energy is maintained within an appropriate range. And, it relates to a designing method for such a heat pipe.
A heat pipe is a device capable of transferring a large amount of heat by circulating a charged and sealed liquid in the pipe. Such a heat pipe has been used for cooling a heating element, such as a semiconductor element mounted on an electronic circuit board.
Recently, the integration of electronic components mounted on a circuit board has been increasingly high. Accordingly, miniaturization and improvement in heat transport capacity for a heat pipe has been requested. For example, in a notebook-type personal computer, a heat pipe requires further reduction in the diameter and the thickness thereof for assembling in the housing of the computer. In addition, to transfer heat to the backside of the liquid crystal panel, the heat pipe requires more flexibility.
A typical type of heat pipe is a wick type heat pipe, in which evaporation and condensation of a charged liquid occurs, and circulation of the condensed liquid to a heat absorbing section (evaporating section) is carried out by the capillary action of a wick or the like.
Here, the above-mentioned wick type heat pipe has the following shortcomings.
(a) There exists an upper limit in the heat transport capacity. The upper limit lowers rapidly when the pipe diameter becomes smaller.
(b) The heat pipe has a complicated and specialized inside structure for circulating the charged liquid.
(c) The heat transport capacity is easily affected by non-condensational gas concentration in the liquid.
(d) Since the wick type heat pipe is a passive device (a device which drives without external power), it is difficult to drive under a top heat condition (a condition in which the heat absorbing section is positioned at the upper position along the gravity direction) or a small gravity field condition.
Due to those shortcomings of the above-mentioned (a) or (b), it is difficult to reduce the diameter of the wick type heat pipes and also difficult to give more flexibility to them. Accordingly, a new type of heat pipe free from the above shortcomings (a) to (d) is needed.
Under such technical background, an oscillatory flow type heat pipe which effectively transfers heat by an oscillatory flow of the charged liquid is now drawing the attention of the industry. The oscillatory flow type heat pipe is classified into two types, (I) and (II) as mentioned below.
(I) A type in which a phase change in the charged liquid is utilized.
This type of heat pipe has a meandering closed loop tunnel filled with a working liquid and vapor thereof with a certain proportion, and utilizes a self-generated two-phase oscillatory flow and a pulsing flow.
(II) A type in which a phase change of the charged liquid is not utilized.
The heat pipe of this type, in which the diffusion enhancing effect in a forced oscillatory flow is utilized, is called as a forced oscillatory flow type heat pipe. The forced oscillatory flow type heat pipe is further classified into two types; a synchronized phase type (so-called a dream pipe) and an inverted phase type (COSMOS (Counter- Stream- Mode- Oscillating- Flow) type).
In the dream type heat pipe, the heat pipe body is constructed of a capillary bundle, and phases of oscillatory flows of the charged liquid in the adjacent capillaries are synchronized. Such a dream type heat pipe is proposed by Kurzweg-Zhao, Phys. Fluid, 27 (1984), 2624-2627.
In the COSMOS type heat pipe, the heat pipe body is constructed of a meandering closed loop flow path, and phases of the oscillatory flows of the charged liquid in the adjacent passages of the meandering flow path are inverted. The COSMOS type heat pipe has a higher response to the top heat condition than a wick type heat pipe or the phase change type heat pipe does. Furthermore, since the heat transport capacity of the COSMOS type heat pipe can be controlled by changing the amplitude and the frequency of the oscillatory flow, it has an advantage that a temperature of a heat radiating body can be also controlled. Moreover, the heat pipe may have a smaller diameter and higher flexibility.
However, the COSMOC type heat pipe capable of practical use is not developed.
The object of the present invention is therefore to provide a forced oscillatory flow type heat pipe in which a ratio (motion coefficient) of the heat transport capacity to the oscillating energy is selected in an appropriate range. And, it is also to provide a designing method for such a heat pipe.
In order to solve the above-mentioned problems, a forced oscillatory flow type heat pipe according to the present invention has a heat pipe body with a closed loop flow path which meanders between a heat absorbing section and a heat radiating section, a liquid charged in said flow path and a mechanism causing an oscillatory flow of said charged liquid, wherein phases of said oscillatory flows of said charged liquid in the adjacent passages of said meandering flow path being inverted, wherein said charged liquid has Prandtle number Pr of less than 100 and modified Womersley number xcex1 of 0.4 to 7, and the Womersly number being defined as
xe2x80x83xcex1=H(xcfx89/xcexa)1/2
where H(m) is half of a width of said flow path; xcfx89(1/s) is angle frequency of oscillation; and xcexa(m2/s) is heat diffusion coefficient of the charged liquid.
A method for designing a forced oscillatory flow type heat pipe according to the present invention is for designing a forced oscillatory flow type heat pipe which has a heat pipe body with a closed loop flow path which meanders between a heat absorbing section and a heat radiating section, a liquid charged in said flow path and a mechanism causing a oscillatory flow of said charged liquid, wherein phases of said oscillatory flows of said charged liquid in the adjacent passages of said meandering flow path being inverted, wherein said charged liquid has Prandtle number Pr of less than 100 and modified Womersley number xcex1 of 0.4 to 7, and the Womersly number being defined as
xcex1=H(xcfx89/xcexa)1/2,
where H(m) is half of a width of said flow path; xcfx89(1/s) is angle frequency of oscillation; and xcexa(m2/s) is heat diffusion coefficient of the charged liquid.
According to the present invention, a means in which a ratio (motion coefficient) of the heat transport capacity to the oscillating energy is maintained within an appropriate range is provided to promote practical application and generalization for the forced oscillatory flow type heat pipe.
Also, it is preferable that the forced oscillatory flow type heat pipe according to the present invention has said Prandtle number Pr of less than 50 and said modified Womersely number xcex1 of 0.4 to 7. Also, it is preferable that specific heat of said charged liquid is higher than 100 j/kg.K.